Second wind is a phenomenon in endurance sports, such as marathons or road running, whereby an athlete who is out of breath and too tired to continue, finds the strength to press on at top performance with less exertion. The feeling may be similar to that of a "runner's high", the most obvious difference being that the runner's high occurs after the race is over. In muscle glycogenoses, an inborn error of carbohydrate metabolism impairs either the formation or utilization of muscle glycogen. As such, those with muscle glycogenoses do not need to do prolonged exercise to experience "hitting the wall". Instead, signs of exercise intolerance, such as an inappropriate rapid heart rate response to exercise, are experienced from the beginning of an activity, and some muscle GSDs can achieve second wind within about 10 minutes from the beginning of the aerobic activity, such as walking. (See below in pathology).